To make the invention more clearly understood, and merely by way of example, the application to the fabrication of an electronic image sensor will be described more precisely. An electronic image sensor is typically intended for example to form the core of a digital camera. It converts an image projected on its sensitive surface into electrical signals.
Conventionally, the image sensor is a monolithic electronic integrated circuit formed on a silicon substrate. Formed on the surface of this substrate are, on the one hand, a matrix of photodetectors and, on the other hand, peripheral electronic circuits called “drivers”. The drivers are used to drive the matrix and extract and process the signals which are output by the matrix and which represent, in electronic form, the image projected on the matrix by an objective.
The monolithic integrated circuit is therefore intended to be combined, in the camera, with a projection optic (a lens or several superposed lenses) that projects the image to be detected on the photosensitive surface of the matrix. This projection optic may form part of the sensor, that is to say it may be integrated with the sensor placed in front of the upper surface thereof, or it may be separate and form part of the camera. However, even if it is separate, it may be useful to provide, on the sensor itself, another optical element, such as a transparent protective wafer or optical filter, mounted directly on the sensor during its fabrication. This is the case not only for photography in the visible range, but it is also for example the case in the field of X-ray imaging, in which it may be desirable to mount, on the sensor, during fabrication of the latter, a wafer of scintillator material, and optically a fiber optic wafer between the scintillator and the matrix of photodetectors.
Thus, examples may be given in which it is useful to combine, with an integrated circuit having electronic functions, a mechanical or optical structure that does not initially form part of the same substrate as the actual integrated circuit, but which has to be attached to this substrate during its fabrication. The simplest example is the transparent wafer on an image sensor, and the explanation will continue with regard to this simple case, although the invention applies to the other structures mentioned above (lenses, fiber wafers, etc.) or even a simple protective cover, whether transparent or not, or a superposition of electronic circuit plates. It will be seen that this transfer of a protective plate or of another structure onto the integrated circuit has hitherto posed problems, and the present invention is aimed at solving these problems.
Conventionally, the fabrication of an integrated circuit is carried out collectively starting from a silicon wafer. A multiplicity of identical individual integrated circuits, arranged in an array of rows and columns, is formed on this wafer by successive operations, namely deposition of layers, photolithography of these layers and chemical etching or ion etching. Next, the wafer is diced into individual “chips”, each corresponding to a single integrated circuit. Each chip is subsequently mounted in an individual package.
When a transparent glass plate has to be attached to the front face of the integrated circuit, before the latter is mounted in its individual package, this plate is firstly cut to the desired dimensions (slightly smaller than those of the chip itself) and is then bonded to the upper face of the chip. In what follows, the term “upper face” or “front face” of the chip or of the substrate refers to the face on which the circuit features defining the electronic functions of the image sensor are formed by successive deposition and etching operations. Among these features, there are contact pads for electrical connection with the outside of the sensor, for controlling the sensor and for receiving the signals representing the image. These pads are located on the periphery of the chip and must remain accessible so as to be able to be soldered to connection pins on the package. This is why the transparent glass plate is cut to smaller dimensions than the surface of the chip—the pads on the periphery of the chip must not be covered by the glass plate.
This fabrication technique means that the transparent wafer must be individually bonded to each chip. The bonding step is therefore not a collective step for the entire wafer of multiple chips. To reduce the fabrication costs, it would be desirable for this step to be a collective step. In addition, if the bonding step is carried out collectively before dicing, the transparent plate protects the chip from dust resulting from the dicing operation, something which is not the case when the bonding takes place only after dicing.
However, a collective fabrication technique encounters a fundamental difficulty: if a glass plate is bonded to the wafer, this glass plate will cover all of the chips, that is to say not only the photosensitive matrix and its drivers, but also the contact pads, thereby making access to them difficult.
Collective fabrication techniques for solving this difficulty have already been envisioned. Thus, U.S. Pat. No. 6,040,235 proposes a technique consisting in making, after the transparent plate has been bonded, a trench having oblique sidewalls that cut the pads, followed by deposition of metal in this trench, in contact with the cut portion of the pads. This metal deposited in the trench is up to the upper face of the plate, so as to form new pads, which are accessible. This technique is complicated—it uses process steps that are not conventional in microelectronics and establishing contacts inside the trench is not easy.
Another technique consists in arranging for the contact pads to be located on the rear face of the chip, while still being connected, through the thickness of the semiconductor wafer, to the circuits formed on the front face of the chip. This technique has been used in particular for image sensors having a thinned semiconductor substrate. However, passing contacts from the front face of the substrate (the face of which all the elements of the integrated circuit are produced) to the rear face is difficult and requires, on the one hand, deep etching and, on the other hand, deposition and etching treatments on both sides of the substrate, which is not desirable.
In the particular field of micromachined electromechanical sensors, for example for accelerometers, various bonding techniques have been also proposed for bonding two silicon wafers, such as for example in U.S. Pat. No. 5,668,033. However, these techniques are complicated as they require particular texturing of the two wafers before they are brought into contact.
There is therefore a need for simpler collective fabrication of chips starting from a wafer, which allows a plate to be collectively bonded to the front face of the wafer, while however permitting electrical access to contacts on the sensor via the front face of the latter after the wafer has been divided up into multiple chips.